1. Field of the Invention
The present invention relates to a semi-submersible offshore vessel of a type used for deep water offshore operations such as oil and gas exploration, drilling and production. The invention introduces a novel way of minimizing motions, and primarily the vertical motions of the vessel, in order to reduce metal fatigue in—for example—riser pipe structures. The vessel exhibits a substantially rectangular ring-pontoon, at least four support columns and an upper deck structure positioned upon said support columns. The offshore vessel may for example be provided with hydrocarbon processing equipment and/or accommodation quarters.
2. Description of the Background Art
In deep water offshore operations such as oil and gas (hydrocarbon) exploration, drilling and production, a semi-submersible offshore vessel of the type described above, is connected to sub-sea wellheads and other installations via a system of several so called riser pipes. However, Applicants have determined that the background art suffers from the following disadvantages.
Drilling operations as well as seabed-to-surface transportation of hydrocarbons (referred to as “production”) are effected through such riser pipes. Since these vessels often operate at considerable depths, the riser pipes of considerable length—often several thousand meters long—are used. Production riser pipes are often made of steel, so called Steel Catenary Risers (SCR), and are sensitive to metal fatigue as the pipes are subjected to forces and motions caused primarily by the wave excited vertical motions of the semi-submersible offshore vessel.
Several designs adapted to primarily minimize vertical motions of offshore vessels are previously known. These designs, however, concentrate on minimizing the vertical motion of the vessel in general, the vertical motion generally being the predominant sea-induced motion in deep sea operational areas with long wave period ranges above 10 seconds. The applicants have found that the greatest problems with riser pipe fatigue are encountered at shorter wave period ranges below 7–8 seconds.